The present invention relates generally to optimized nickel hydroxide positive electrode materials, a process for fabricating such materials, positive electrodes fabricated using such materials and nickel metal hydride (xe2x80x9cNiMHxe2x80x9d) batteries incorporating such materials. More specifically, this invention relates to a multi-element nickel hydroxi de positive electrode material characterized by an engineered activation energy and optimized crystallite size. Preferably this is accomplished by the incorporation of modifier elements into the bulk thereof in a single chamber reactor so as to provide a NiMH battery exhibiting multiple electron transfer, improved capacity, high temperature performance, and cycle life.
There are m any known types of Ni based rechargeable alkaline cells such as nickel cadmium (xe2x80x9cNiCdxe2x80x9d), NiMH, nickel hydrogen, nickel zinc, and nickel iron (xe2x80x9cNiFexe2x80x9d) cells. At one time NiFe and then NiCd batteries were the most widely used. Just as NiFe batteries were displaced by NiCd batteries, NiCd batteries have now been steadily replaced in all applications by NiMH cells. Compared to NiCd cells, NiMH cells made of synthetically engineered materials have superior electrochemical performance parameters, such as specific energy and energy density, and contain no toxic or carcinogenic elements, such as Cd, Pb, and Hg. For purposes of this patent application, the terms xe2x80x9cbatteriesxe2x80x9d and xe2x80x9ccellsxe2x80x9d will be used interchangeably when referring to one cell, although the term xe2x80x9cbatteryxe2x80x9d can also refer to a plurality of electrically interconnected cells.
While the present discussion focuses on NiMH batteries, it should be understood that the modified nickel hydroxide materials of the present invention can be used in all types of batteries using nickel hydroxide based positive electrode materials. The term xe2x80x9cutilizationxe2x80x9d will be employed in this disclosure to describe the instant,invention in the manner well accepted by those ordinarily skilled in the electrochemical art. As used herein xe2x80x9cutilizationxe2x80x9d will refer to the percentage of the electrons of the nickel hydroxide positive electrode electrochemically transferred during the charge/discharge cycling of the electrode relative to the total number of nickel atoms present in the nickel hydroxide material.
In general, NiMH cells employ a negative electrode made of hydrogen storage alloy that is capable of the reversible electrochemical storage of hydrogen. NiMH cells also employ a positive electrode made from nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. Upon application of an electrical potential across a NiMH cell, water is dissociated into one hydroxyl ion and one hydrogen ion at the surface of the negative electrode. The hydrogen ion combines with one electron and diffuses into the bulk of the hydrogen storage alloy. This reaction is reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The development of commercially viable NiMH batteries began in the 1980s by improving the negative electrode materials by making them xe2x80x9cdisorderedxe2x80x9d as taught by Ovshinsky, et al in U.S. Pat. No. 4,623,597. Such disordered negative electrode materials represented a total departure from other teachings of the period that urged the formation of homogeneous and single phase negative electrodes. (For a more detailed discussion, see U.S. Pat. Nos. 5,096,667; 5,104,617; 5,238,756; 5,277,999; 5,407,761 and 5,536,591 and the discussion contained therein. The disclosure of these patents are specifically incorporated herein by reference.)
The use of disordered negative electrode metal hydride materials significantly increases the reversible hydrogen storage characteristics required for efficient and economical battery applications, and results in the commercial production of batteries having high density energy storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
As discussed in U.S. Pat. No. 5,348,822, nickel hydroxide positive electrode material in its most basic form has a maximum theoretical specific capacity of 289 mAh/g, when one charge/discharge cycles from a xcex2II phase to a xcex2III phase and results in one electron transferred per nickel atom. It was recognized in the prior art that greater than one electron transfer could be realized by deviating from the xcex2II and xcex2III limitations and cycling between a highly oxidized xcex3-phase nickel hydroxide phase and either the xcex2II phase and/or the "khgr"-phase. However, it was conventionally recognized dogma that such gamma phase nickel hydroxide formation destroyed reversible structural stability and therefore cycle life was unacceptably degraded. A large number of patents and publications in the technical literature disclosed modifications to nickel hydroxide material designed to inhibit and/or prevent the destructive formation of the transition to the xcex3-phase.
Attempts to improve nickel hydroxide positive electrode materials began with the addition of elements to compensate for what was perceived as the inherent problems of the material. The use of compositions such as NiCoCd, NiCoZn, NiCoMg, and their analogues are described, for example, in the following patents:
U.S. Pat. No. Re. 34,752, to Oshitani, et al., reissued Oct. 4, 1994, describes a nickel hydroxide active material that contains nickel hydroxide containing 1-10 wt % zinc or 1-3 wt % magnesium to suppress the production of xcex3-NiOOH. The invention is directed toward increasing utilization and discharge capacity of the positive electrode. Percent utilization and percent discharge capacity are discussed in the presence of various additives.
Oshitani, et al. describe the lengths that routineers in the art thought it was necessary to go to in order to prevent the presence of substantial amounts of xcex3-NiOOH. The patent states:
Further, since the current density increased in accordance with the reduction of the specific surface area, a large amount of higher oxide xcex3-NiOOH may be produced, which may cause fatal phenomena such as stepped discharge characteristics and/or swelling. The swelling due to the production of xcex3-NiOOH in the nickel electrode is caused by the large change of the density from high density xcex2-NiOOH to low density xcex3-NiOOH. The inventors have already found that the production of xcex3-NiOOH can effectively be prevented by addition of a small amount of cadmium in a solid solution into the nickel hydroxide. However, it is desired to achieve the substantially same or more excellent effect by utilizing additive other than the cadmium from the viewpoint of the environmental pollution.xe2x80x9d
U.S. Pat. No. 5,366,831, to Watada, et al., issued Nov. 22, 1994, describes the addition of a single Group II element (such as Zn, Ba, and Cd) in a solid solution with nickel hydroxide active material. The Group II element is described as preventing the formation of gamma phase nickel hydroxide thereby reducing swelling, and the cobalt is described as reducing the oxygen overvoltage thereby increasing high temperature charging efficiency. Both oxygen overvoltage and charge efficiency are described as increasing with increasing cobalt.
U.S. Pat. No. 5,451,475, to Ohta, et al., issued Sep. 19, 1995, describes the positive nickel hydroxide electrode material as fabricated with at least one of the following elements added to the surface of the particles thereof: cobalt, cobalt hydroxide, cobalt oxide, carbon powder, and at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y. The cobalt, cobalt compound, and carbon are described as constituents of a conductive network to improve charging efficiency and conductivity. The powdery compound is described as adsorbed to the surface of the nickel hydroxide active material where it increases the overvoltage, for evolution of oxygen, thereby increasing nickel hydroxide utilization at high temperature. Ohta, et al. claims that increased energy storage in NiMH cells. using the disclosed invention remains constant up to a high number of charge/discharge cycles and capacity does not drop as much at higher temperatures as it does in cells that do not embody the invention.
U.S. Pat. No. 5,455,125 to Matsumoto, et al., issued Oct. 3, 1995, describes a battery having a positive electrode comprising nickel hydroxide pasted on a nickel foam substrate with solid solution regions of Co and salts of Cd, Zn, Ca, Ag, Mn, Sr, V, Ba, Sb, Y, and rare earth elements. The addition of the solid solution regions is intended to control the oxygen overvoltage during charging. The further external addition of xe2x80x9celectric conducting agentsxe2x80x9d such as powdered cobalt, cobalt oxide, nickel, graphite, xe2x80x9cand the like,xe2x80x9d is also described. Energy density is shown as constant at 72 Wh/kg at 20xc2x0 C. and 56 Wh/kg at 45xc2x0 C. for embodiments of the invention over the life of the NiMH cell.
U.S. Pat. No. 5,466,543, to Ikoma, et al., issued Nov. 14, 1995, describes batteries having improved nickel hydroxide utilization over a wide temperature range and increased oxygen overvoltage resulting from the incorporation of at least one compound of yttrium, indium, antimony, barium, or beryllium, and at least one compound of cobalt or calcium into the positive electrode. Cobalt hydroxide, calcium oxide, calcium hydroxide, calcium fluoride, calcium peroxide, and calcium silicate are specifically described compounds. Additionally described additives are cobalt, powdery carbon, and nickel. The specification particularly describes AA cells using a positive electrode containing 3 wt % zinc oxide and 3 wt % calcium hydroxide as superior in terms of cycle life (250 cycles at 0xc2x0 C., 370 cycles at 20xc2x0 C., and 360 cycles at 40xc2x0 C.) and discharge capacity (950 mAh at 20xc2x0 C., 850 mAh at 40xc2x0 C., and 780 mAh at 50xc2x0 C.).
U.S. Pat. No. 5, 489,314, to Bodauchi, et al., issued Feb. 6, 1996, describes mixing the nickel hydroxide positive electrode material with a cobalt powder compound followed by an oxidation step to form a beta cobalt oxyhydroxide on the surface of the nickel hydroxide powder.
U.S. Pat. No. 5,506,070, to Mori, et al., issued Apr. 9, 1996, describes nickel hydroxide positive electrode material containing 2-8 wt % zinc mixed with 5-15 wt % cobalt monoxide. The zinc reduces swelling and the cobalt increases utilization. The capacity of the resulting electrode is stated as being xe2x80x9cimproved up to 600 mAh/ccxe2x80x9d without further description.
U.S. 5,571.636, to Ohta, et al., issued Nov. 5, 1996, describes the addition of at least one powdery compound of Ca, Sr, Ba, Cu, Ag, and Y to the surface of nickel hydroxide active positive electrode material. This patent states that these compounds are adsorbed to the surface of the nickel hydroxide active material creating a conductive network that increases the oxygen overvoltage and improves utilization of the active material at high temperatures. Increased capacity in NiMH cells using the ""636 invention remains constant up to a large number of cycles and utilization does not drop as much at higher temperatures as it does in cells that do not embody the invention.
In all of the prior art, the basic nickel hydroxide material is treated, most commonly, by the addition of a single element or a compound thereof, usually Co, to increase conductivity and usually one other element or a compound thereof, usually Cd or Zn, to suppress and/or prevent y-phase formation. The mechanisms for the asserted improvements in all the above patents are attributable to the following effects,
1. Improved speed of activation, resistance to poisons, and marginal capacity improvement via increased utilization. At the present time, most commercial nickel metal hydride batteries achieve these effects through the addition of up to 5 wt % cobalt. A noted researcher, Delmas, in the Proceeding of the Symposium on Nickel Hydroxide Electrode 118-133 (1991) observed that much higher capacity could result if as much as 20% trivalent cobalt was used. However, even setting environmental and cost considerations aside, the addition of 20% Co is unstable and thus not applicable to commercially viable systems. Frequently, powdered carbon, powdered cobalt metal, and powdered nickel metal are externally also added to create separate conductive networks and thereby improve utilization. Of course, a major drawback of increasing the amount of such elements that are added is that the amount of active nickel hydroxide electrode material is correspondingly reduced, thereby reducing capacity of the electrode. Further, since Co is very expensive, the addition of Co increases cost.
2. Cycle life is extended by decreasing swelling that is initiated by density changes between the oxidized and reduced states of the nickel hydroxide material. Swelling, in turn, is accelerated by the uncontrolled density changes between xcex2II-xcex2III phase nickel hydroxide and xcex1-xcex3xcex2 or xcex2II-xcex3 phase nickel hydroxide. Cd and Zn incorporated into the nickel hydroxide effectively reduce the swelling by reducing the difference in density in the charged and discharged material and increasing the mechanical stability of the nickel hydroxide material itself. This is accomplished by promoting oxygen evolution and thereby reducing charge acceptance which prevents the nickel hydroxide material from attaining the highly oxidized state (the xcex3-phase state). However, by suppressing or at least significantly inhibiting xcex3-phase state formation, the nickel hydroxide is limited to low utilization. Further, in order to effectively inhibit xcex3-phase nickel hydroxide, it is necessary to employ a relatively high wt % of the inhibitor element such as Zn, which high percentage results in a greatly reduced amount of active material being present thereby resulting in reduced electrochemical capacity.
3. The aforementioned xe2x80x9csafety releasexe2x80x9d mechanism of oxygen evolution to avoid highly oxidized states (xcex3-phase) of nickel hydroxide material actually is an impediment to high temperature operation because a significant increase in the rate of oxygen evolution occurs with increasing temperature. The effect of such increased oxygen evolution is a very substantial decrease in utilization and ultimately a reduction in energy storage at higher temperatures in the NiMH battery using these materials. At 55xc2x0 C., for example, run times of a battery may be reduced by 35-55% compared to the room temperature performance of that same battery.
Elevated operational temperature conditions aside, none of these modifications of the active positive electrode material suggested by the prior art result in more than an incremental improvement in performance and none result in a significant increase in the capacity and/or utilization of the nickel hydroxide material itself, even at room temperature. All prior art batteries are limited to less than one electron transfer per nickel atom. Further, these modifications fail to address the special operational requirements of NiMH batteries, particularly when NiMH batteries are used in electric vehicles, hybrid vehicles,. scooters and other high capacity, high drain rate applications. Because NiMH negative electrodes have been improved and now exhibit an extremely high storage capacity, the nickel hydroxide positive electrode material is essentially the limiting factor in overall battery capacity. This makes improving the overall electrochemical performance of the nickel hydroxide material in all areas more important than in the past. Unfortunately, the elements currently added or previously suggested to be added to the nickel hydroxide material result in insufficient improvements in performance before competing deleterious mechanisms and effects occur. For example, Cd cannot be used in any commercial battery because of the environmental impact thereof, and Co and Zn appear to become most effective only at levels that result in a significant decrease in cell capacity; more specifically, energy per electrode weight.
Accordingly, there remains a long felt need in the art for an improved, higher capacity, higher utilization, high temperature nickel hydroxide positive electrode for a nickel metal hydride battery.
One object of the present invention is to provide a nickel modified hydroxide positive electrode material having improved capacity, cycle life, rate capability, utilization and high temperature performance.
Another object of the present invention is to provide a nickel hydroxide positive electrode material exhibiting improved electrochemical performance attained by the addition of modifier elements throughout the bulk thereof and at levels that avoid the deleterious effects seen in the prior art while providing modified activation energy, higher electrical conductivity and improved utilization.
These and other objects of the present invention are satisfied by a high capacity, long cycle life positive electrode material for use in an alkaline rechargeable electrochemical cell comprising: nickel hydroxide active material internally containing at least three modifier elements, the nickel hydroxide having a modified hydrogen chemical potential and reduced activation energy providing improved conductivity within and throughout the active material itself and greater use of the available storage sites through improved proton transport.
Other objects of the present invention are satisfied by a high capacity, long cycle life positive electrode material for use in an alkaline rechargeable electrochemical cell comprising: nickel hydroxide; and at least three and preferably four modifiers incorporated throughout the bulk of the nickel hydroxide material. These four modifiers are most preferably Ca, Co, Mg, and Zn.
A still further object of the present invention is to provide doped and alloyed nickel hydroxide active materials having improved charge efficiencies, especially at elevated temperatures. This is accomplished by modifying the oxygen evolution potential (oxygen overvoltage) and improving the resistance of the nickel hydroxide material to unavoidable impurities present in NiMH batteries which can promote premature oxygen evolution.
Yet another object of the instant invention is to provide high electrochemical capacity in sealed, starved electrolyte, NiMH cells. This is accomplished via the use of a mixed xcex2-phase and xcex3-phase nickel oxyhydroxide material on charge (having in the range of 5-25% or more xcex3-phase present), which xcex3 phase is introduced in a manner that does not destroy cycle life (as repeatedly disclosed in the prior art). This is accomplished by incorporating the xcex3-phase during initial battery formation and not allowing the xcex3-phase to gradually form by xe2x80x9caccidentxe2x80x9d during cycling.
It is to be noted that the term xcex3-phase nickel oxyhydroxide refers to a highly oxidized state of a base nickel hydroxide material obtained via any of several alternate routes One route is conventional xcex3-phase NiOOH where the oxidation state of the nickel is higher than 3+ during charging. This may be accomplished by inserting some anions, such as NO3xe2x88x92 among the water layer separating the adjacent Ni-O planes. For example, the average oxidation number of Ni in NiOOHxe2x80x94(H2O)x(NO3)0.5 is 3.5+. Another route of achieving xcex3-phase NiOOH is an innovation described in this patent, i.e., doping the host Ni(OH)2 with a metal having a lower oxidation number such as Mg2+ and Ca2+. The doping effect of these cations is identical to that of the anions added via the first route, i.e., to push Ni to a higher oxidation state. For example, the average oxidation number of Ni in NiMg0.5 (OOH)1.5 is 3.5+.
There are several possible phase changes that occur during the charge/discharge cycling of nickel hydroxide material. The most common one is xcex2(II)/xcex2(III) transformation will provide for a single electron transfer per Ni atom (Commonly, this Ni utilization is further reduced in practice to 0.8-0.9 in starved electrolyte sealed cells.) Then there is a conventional xcex1-Ni(OH)2/xcex3-NiOOH transformation which involves moving some anions among the water layer between Nixe2x80x94OH planes with a large change in the c lattice constant of the unit cell; and more than one electron transfer per Ni atom. The large change in the c lattice constant promotes easy pulveration and inferior cycling performance. This is the main reason that all prior art tried to inhibit the formation of xcex3-NiOOH to optimize the cycling performance. Of course, the present inventors have shown that gradual unrestrained growth of large pockets of xcex3-NiOOH can indeed cause detrimental swelling, but that xe2x80x9cbuilt in,xe2x80x9d localized, finely distributed xcex3-NiOOH in fact provides excellent cycle life. The third possible transformation as taught in this patent application is directly changing from xcex2Ni(OH)2 to xcex3-NiOOH during the charge. process, which includes moving both water molecules and anions in and out of the structure, and contributes an even greater lattice constant change during cycling. The fourth possibility is forming the higher oxidation state of Ni by substituting lower oxidation state cations for some Ni atoms thereby creating a variation in local binding environments. In the last case, there will be no need to move water or any other anions around and the disturbance to the. lattice constant will be small, no greater than the change that occurs during a routine xcex2(II)/xcex2(III) transformation. Note that these phase transitions, as described above, provide additional hydrogen sites and promote faster proton transport.
A still further object of the present invention is to provide a method by which modifier elements, such as Ca, can be incorporated internally into highly modified nickel hydroxide material. As discussed above. Ca has commonly been added externally to the Nixe2x80x94Coxe2x80x94Zn nickel hydroxide positive electrode material to improve high temperature performance of the battery. Heretofore, the simultaneous precipitation of Ca has been avoided during nickel hydroxide fabrication because the starting charge metals are incorporated into a metal sulfate solution for reaction with an alkali; such as NaOH. Ca added internally, whether alone or in combination with other xcex3-phase suppressant elements such as Zn and Ca, to nickel hydroxide has been disclosed by Oshitani, et al as a xcex3-phase suppressant. Nixe2x80x94Coxe2x80x94Zn. Nixe2x80x94Coxe2x80x94Ca, Nixe2x80x94Coxe2x80x94Mg were all shown to inhibit the formation of xcex3-phase, with Zn being much more effective than Ca or Mg. In fact, Nixe2x80x94Coxe2x80x94Zn has become the commercial positive electrode material of choice. Where 3 atomic % Zn can effectively inhibit xcex3-phase, it is necessary to use upwards of 10 wt % Ca or Mg in order to accomplish this same task. No one has taught, disclosed or suggested that the simultaneous coprecipitation of highly modified compositions which do not inhibit xcex3-phase formation Nixe2x80x94Coxe2x80x94Znxe2x80x94Mgxe2x80x94Ca, would produce the beneficial effect of high utilization, long life, and improved high temperature performance. Since Ca is not soluble in NiSO4, the subject inventors developed a dual reactant feed approach which can add elements such as Ni, Co, Zn, Mg, Cu, Mn via a metal sulfate (xe2x80x9cMeSO4xe2x80x9d) solution. A special feed of Ca reactant, such as CaNO3 or CaCl2, is the vehicle for the simultaneous internal Ca addition. This approach is vastly superior in that Ca is thus placed in intimate proximity on an atomic basis, whereas in conventional high surface area Ni(OH)2 (xcx9c10 m2/gram), externally added Ca can never reach all necessary internal atomic locations.
Still another aspect of the present invention is to provide high capacity, long cycle life positive electrode material for use in an alkaline rechargeable electrochemical cell comprising: nickel hydroxide active material containing at least three modifier elements, the modified nickel hydroxide having a modified activation energy and chemical potential providing inherently higher conductivity nickel hydroxide active material. This improved conductivity allows greater utilization of hydrogen storage sites in a xcex2-II/xcex2-III phase transition and/or improved formation and distribution of high capacity xcex3-phase regions.
Another object of the present invention is to provide high capacity, long cycle life positive electrode material for use in an alkaline rechargeable electrochemical cell comprising: nickel hydroxide particles that are spherical and average 5-20 xcexcm in size, the particles formed of fine crystallites typically averaging 70-150 xc3x85 in size. This typical size is significant in that the number of storage sites on the surface of the crystallites approximates the number of storage sites in the bulk, thereby resulting in heretofore unaccessible sites being made available for hydrogen storage. Such small crystallite sizes allow for better diffusion of ions and electrolyte in the bulk of the material.
A final object of the present invention is to provide a method of making a high capacity, long cycle life nickel hydroxide positive electrode for use in an alkaline rechargeable electrochemical cell comprising: combining MeSO4, MeNO3, and/or MeCl2, NH4OH and NaOH in a single reactor, maintaining the reactor at a constant temperature of 20-100xc2x0 C., agitating the combination at a rate of 400-1000 rpm, and controlling the pH of the agitating combination at 6-10. This unique, single reactor method permits the incorporation of multi-element modifiers that cannot be incorporated using a MeSO4 feed stream as described in the prior art. This method also permits a previously unattainable uniform distribution of the modifiers in the bulk of the nickel hydroxide matrix, while maintaining excellent tap density, crystallite size, and surface area in a high yield, cost effective, commercially viable single precipitation process reactor.